Ramblings from another nerd on the grid
Active Directory Federation Services is a highly secure, highly extensible, and Internet-scalable identity access solution that allows organizations to authenticate users from partner organizations. Using AD FS in Windows Server 2008, you can simply and very securely grant external users access to your organization’s domain resources. AD FS can also simplify integration between untrusted resources and domain resources within your own organization.
Active Directory Federation Services 2.0 helps IT enable users to collaborate across organizational boundaries and easily access applications on-premises and in the cloud, while maintaining application security.
Go get it @ http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=118c3588-9070-426a-b655-6cec0a92c10b.